Carbon molecular sieves are porous substrates with an open-network structure of controlled molecular dimension which may be used to separate mixtures of small (i.e. in diameter, weight or shape) molecular gases or liquids from larger molecular sized gases or liquids based on a difference in molecular size or a difference in diffusivity rates. See for example, Grant, U.S. Pat. No. 3,884,830 and references cited therein.
Carbon molecular sieves have been prepared from a variety of substrates by a number of different manufacturing processes. See for example, Mason et al., U.S. Pat. No. 3,222,412, employing anthracite coal, elevated temperatures and an inert atmosphere; Munzner et al., U.S. Pat. No. 3,801,513, employing coke or coconut shells, elevated temperatures and pore blocking by carbon deposition; Yuki, U.S. Pat. No. 4,046,709 employing vinylidene chloride copolymers and elevated temperatures; and Vesterdal, U.S. Pat. No. 2,556,859 employing bituminous coal or coconut shells, elevated temperatures and an inert atmosphere.
Carbon molecular sieves have generally been prepared in an externally fired rotary kiln or other similar batch-type furnace. The major difficulties associated with the noncontinuous batch-type manufacture of carbon molecular sieves are (1) the difficulty of maintaining furnace atmospheric and temperature limits for the control of sieve pore diameter and (2) the product variability from batch to batch, i.e., quality control. See Munzner et al., U.S. Pat. No. 3,962,129.
Carbon molecular sieve average effective pore diameters are generally governed by the following factors:
(A) furnace temperature PA0 (B) furnace atmosphere PA0 (C) residence time PA0 (D) presence or absence of a pore blocking substance.
Control of the first two conditions has been a major problem aassociated with the prior art noncontinuous, batch-type manufacturing processes.
The use of a continuous feed type heating means, as described herein, overcomes the difficulties associated with atmosphere and temperature control of noncontinuous, batch processes, thereby assuring pore diameter selection and overall sieve quality control. The instant process also generates carbon sieves with better selectivity and capacity values than prior art batch processes.